According to most existing telecommunications standards, the transmission of speech information over a wireless interface takes the form of compressed speech parameters. Upon receipt of compressed speech parameters at a base station in communication with a mobile unit, the speech parameters are processed by a codec (coder/decoder), which converts (expands) the speech parameters into speech samples, typically at a rate of 64 kilobits per second (kb/s) in order to provide compatibility with the public switched telephone network (PSTN). The speech samples at 64 kb/s are then transmitted over the PSTN towards the called party. The speech samples associated with a given call may share the same link as speech samples associated with other calls by virtue of time division multiplexing (TDM), which provides for fixed-duration time slots to be allotted to individual calls.
If the called party is connected directly to the PSTN, such as via a wireline connection, the speech samples having travelled through the network will simply be converted into audio form by a digital telephone unit at the called party site. Of course, the called party may also be a second mobile unit, in which case the speech samples will terminate at a second base station, where a second codec re-converts the speech samples back into compressed speech parameters for transmission to the second mobile unit via a wireless interface. The usage of a source decoder to expand speech parameters into a stream of speech samples, in combination with the use of a destination encoder for re-compression of these samples into a second set of compressed speech parameters, is referred to as operation of codecs in tandem, or “tandem operation”.
Those skilled in the art will appreciate that when both the called and calling parties are mobile units, the tandem operation described above introduces a degradation in service quality, as errors may be introduced by the decompression and re-compression operations performed by the source and destination codecs, respectively. Such error should in principle be avoidable, as neither codec operation is required by virtue of the second base station requiring the compressed speech parameters rather than the expanded speech samples. Thus, it is of interest to find a solution to the problem of service quality in call connections involving tandem codecs.
Two classes of solutions to the problem relating to the service quality in call connections involving tandem codecs have already been described and standardized, or are well in their way towards standardization. The earlier of the two methods, called Tandem-Free Operation (TFO), uses an in-band handshaking protocol to detect the presence of tandem codecs, and then proceeds to insert the compressed speech parameters within the 64 kb/s sample stream. This arrangement bypasses the requirement for decompression at the source codec and (re-)compression at the destination codec, which obviates the occurrence of errors at these two stages. As a result, a high quality of service can be achieved for a given end-to-end call between two mobile units. However, the standardized TFO approach provides no bandwidth advantage, as the full bandwidth ordinarily needed for the 64 kb/s sample stream is consumed for transmission of the compressed speech parameters.
A more recent approach, called Transcoder-Free Operation (TrFO), uses out-of-band signaling to detect call scenarios involving tandem codecs at call set-up time. Thereupon action is taken to put in place a direct end-to-end link to provide for a direct exchange of the compressed speech parameters without the involvement of network transcoders. However, while it provides for a savings and resource reduction compared to the standardized TFO approach, the TrFO implementation suffers from the disadvantage of added cost and complexity due to, for example, the requirement for out-of-band signaling.
For more information on the TFO and TrFO techniques, the reader is invited to refer to the following documents that are hereby incorporated by reference:                3rd generation partnership project, Technical specification group core network, Out of band transcoder control—Stage 2 (3GPP TS 23.153 V4.4.0 (2001-12));        3rd generation partnership project, Technical specification group core network, Bearer-independent circuit-switched core network, Stage 2 (3GPP TS 23.205 V4.4.0 (2002-03));        3rd generation partnership project, Technical specification group (TSG) RAN3, Transcoder free operation (3GPP TR 25.953 V4.0.0 (2001-03));        3rd generation partnership project, Technical specification group services and system aspects, Inband tandem free operation (TFO) of speech codecs, service description—Stage 3 (3GPP TS 28.062 V5.0.0 (2002-03));        
It will thus be apparent that there is a need in the industry to provide a solution that is as robust and easy to implement, while also providing bandwidth and resource savings.
Moreover, the use of conventional codec-bypass schemes such as TFO has heretofore been limited to enhancing the quality of calls established between two suitably enabled base station units in a mobile-to-mobile call. When one party is not so enabled, e.g., a telephone connected to a common packet-switched network via a network gateway, the use of conventional codec-bypass techniques is not possible. It would therefore be an advantage to exploit the ability of one party's codec-bypass capabilities, even when the other party is not a suitably enabled base station unit.
In addition, the use of conventional codec-bypass schemes is often limited by the use of backhaul gateways in a network, even when both parties to a call are codec-bypass-enabled base station units. Such gateways compress speech samples into a different format prior to transmittal of the formatted speech samples over a network. Unfortunately, when codec-bypass information is carried within the bit structure of the speech samples, the compression effected by a backhaul gateway results in loss of the information and hence prevents advantageous usage of this facility. Hence, it would be beneficial to be able to allow tandem-free operation in circumstances where a backhaul gateway is used.